MEXICO CITY -- Even when he was playing for Mexico's national team, Edgar Castillo sensed he should be wearing the soccer jersey of the United States, the country of his birth. Despite perfect Spanish, his Mexican teammates playfully called him "El Gringo" -- a nickname that stuck.

Castillo has always stood out, starting at Mayfield High School in Las Cruces, N.M., and he's still hard to miss. His darting moves on the ball belie the fact that he's a defender. And the striking tattoo on his left shoulder of the Virgin of Guadalupe -- Mexico's most revered religious icon -- attests to his mixed roots.

"Even though I wore the Mexico jersey, I had that feeling I should play for the U.S.," Castillo said in an interview with The Associated Press. "I feel confident. They have to call me first, and then after that I know I have the capability of doing things right."

Because of a new rule, and because he holds both U.S. and Mexican passports, the left-footed, left-sided defender may get a chance to showcase his slashing runs for the United States.

The new regulation from FIFA -- the governing body of world soccer -- allows players of any age to switch countries once, provided they have dual nationality and have not played in an official match for the senior national team. The old rule prohibited players 21 and over from switching national teams.

U.S. coach Bob Bradley and United States Soccer Federation officials have declined to say much about Castillo's potential impact. There's the fear of speaking about a player who's yet to take the field for the United States, and the wish to keep the move low-key in Mexico, where newspaper headlines have already described the 22-year-old Castillo as a "traitor" and a "deserter."

"Everything is open," U.S. Soccer Federation president Sunil Gulati said. "I don't think Bob (Bradley) is ready to say that he's coming into the team or not coming into the team. But, from our perspective, we've had some preliminary discussions with Edgar."

Bradley acknowledged last year in an interview that missing out on a player like Castillo was "frustrating."

Castillo could fill a need at left back, a position that was shared in the recent Confederations Cup by Jonathan Bornstein and Carlos Bocanegra, typically a central defender. Even midfielder DaMarcus Beasley was tried there in a recent World Cup qualifier. Heath Pearce and Marvell Wynne Jr. are other possibilities.

Castillo's speed and deft moves on the ball would bring a bonus to the back line -- like a catcher who can run, or a center who can shoot the 3-pointer. His younger brother, Noel, is also a left-footed, left back playing professionally in Mexico.

"I think if you were to talk about one position where the U.S. has been unsettled, it would be left back," said Eddie Rock, Castillo's agent. "Edgar brings qualities that are special and unique, which is the ability to attack out of the back and take people on on the wing."

Assuming Castillo makes the jump to the U.S. team, he wouldn't be eligible when the United States plays at Mexico City in a World Cup qualifier on Aug. 12 at 105,000-seat Aztec Stadium, where the Americans have never beaten Mexico. If he were on the field, it would certainly add fuel to an already heated rivalry.

"I can imagine that -- putting on the U.S. jersey -- and I can imagine the Mexican people's reaction," Castillo said.

Castillo has played for a half-dozen unfamiliar coaches in the last several years. That carousel should end this season with Monterrey club Tigres, where he is united with Daniel Guzman, his old coach at Mexican club Santos Laguna.

By defenders' standards, Castillo is tiny -- only 5-feet-7 and 135 pounds. He's wiry more than strong and has tried to bulk up. A forward for much of his career, he'd still prefer to run at opponents rather than run them down on defense.

"I'm a fast little guy," he said. "I like to go attack but I'm not very good defender yet. I have to work on that. I get in a little trouble for that, but not all the time."

FIFA's new rule could give U.S. soccer another boost. Holding midfielder Jermaine Jones -- another tough position to fill -- has said he will switch from Germany to the United States. Jones, who plays for German Bundesliga club Schalke, is the son of a German mother and American father.

The U.S. Soccer Federation has been working to recruit immigrant, Hispanic and African-American players, an effort president Gulati -- an immigrant himself -- has stressed since taking office three years ago. Castillo is simply one who initially got away.

Castillo said he attended a national camp in Massachusetts when he was 14, thinking it might be a break. At the time he felt ignored. In hindsight, he recognizes there are many talented young American players, and he was only one.

"I thought I was overlooked, but now I hope I get the call up," he said.

Castillo left Mayfield High School during his senior year for a tryout in Mexico's professional league, eventually becoming a starter for Mexican first-division club Santos, which won the 2008 Clausura championship -- one of two league titles decided each year in Mexico.

En route, he got a call-up from Mexico's then-national team coach Hugo Sanchez and played in four exhibition matches, which don't count as official games.

In March, Sven-Goran Eriksson, who succeeded Sanchez, called up Castillo for his first official games -- World Cup qualifiers against Honduras and Costa Rica. But he missed both, unable to attend because he'd lost his Mexican passport. Current Mexico coach Javier Aguirre also hasn't used him in World Cup qualifiers.

Castillo was ripped by the Mexican media over the lost passport, which appears to have been a stroke of fortune. Had he played, he would have been ineligible to switch national teams.

Castillo and his agent said they had no idea the rule change was coming when Castillo lost his passport. He said he never found it and had it replaced.

"I don't know what happened when I lost my passport," Castillo said. "Then the months went by and all of a sudden Eddie (his agent) told me there was a new rule."

Rock said Castillo was "gutted" to have missed the two matches, but said the incident may have changed his career -- and his outlook.

"I think what happened added some maturity to him," said Rock, who hopes Castillo will be eligible for the U.S. in two months. "He took a beating in the press for what happened, and rightfully so."